In long term evolution (LTE) which has been standardized by the 3rd Generation Partnership Project (3GPP) as the 3.9 generation wireless transmission system, a multiple input multiple output (MIMO) technology performing wireless transmission using a plurality of transmission and reception antennas has been specified in order to significantly improve frequency use efficiency from the third-generation wireless transmission method. It is possible to realize improvement of the transmission speed, without increasing the frequency bandwidth, due to a spatial division multiplexing technology that is one of MIMO technologies. In order to achieve a peak transmission rate of 1 Gbps on downlink (referring to communication from a base station apparatus to a terminal apparatus) transmission in the LTE-Advanced (LTE-A) that is the developed version of LTE, a single-user MIMO (SU-MIMO) capable of spatially multiplexing eight streams at maximum is considered. The SU-MIMO is MIMO transmission between a base station apparatus having a plurality of transmission antennas and a single terminal apparatus having a plurality of reception antennas.
The MIMO transmission is roughly divided into closed-loop type MIMO transmission in which a transmission apparatus (base station apparatus) requires channel information (CSI) and open-loop type MIMO transmission in which CSI is not required, and it is reported that the achievable frequency use efficiency in the closed-loop type MIMO is greater than in the open-loop type MIMO. However, in the case of a wireless communication system based on frequency division duplex using different carrier frequencies in the uplink and downlink, it is necessary to feed back the CSI from the terminal apparatus in order for the base station apparatus to achieve the CSI, and thus there is a problem of a significant increase in the overhead.
Thus, in LTE, a codebook-based closed-loop type MIMO transmission capable of significantly suppressing the amount of overhead related to the notification of CSI is supported. In the codebook-based closed-loop type MIMO, a codebook describing a plurality of linear filters is previously shared between the base station apparatus and the terminal apparatus, and the terminal apparatus extracts a desired transmission filter from the codebook described above and notifies the base station apparatus of the number (index). The base station apparatus performs precoding on the transmission data, based on the linear filter of which notification is sent, and then performs MIMO transmission. Since the notification of the CSI is sent based on the codebook, it is possible to more significantly suppress the amount of overhead, as compared to a method in which the terminal apparatus directly quantizes the CSI and notifies the base station apparatus of the CSI.
Further, in NPL 1, for the purpose of suppressing an increase in overhead due to an increase in the number of transmission antennas, a codebook of a double codebook structure is suggested. This realizes MIMO transmission of multiple streams while suppressing an increase in the overhead to the minimum by using two codebooks: a first codebook corresponding to an antenna gain in the same polarized wave, and a second codebook corresponding to a phase difference between the polarized waves on the assumption of using polarized transmission and reception antennas.
Meanwhile, a plurality of terminal apparatuses which are simultaneously connected are regarded as a virtual large-scale antenna array, and LTE supports multi-user MIMO (MU-MIMO) which spatially multiplexes a transmission signal from the base station apparatus to each terminal apparatus. Similar to the SU-MIMO, even in the MU-MIMO, the terminal apparatuses notify the base station apparatus of the desired linear filters from the codebook, and the base station apparatus determines whether to perform the MU-MIMO in which the data addressed to the plurality of terminal apparatuses is spatially multiplexed, based on the linear filters of which notification is sent from the plurality of terminal apparatuses, and transmits the spatially multiplexed data.
However, in the codebook-based MU-MIMO in which the terminal apparatus does not report the CSI itself, an opportunity to perform spatial multiplexing is not significantly improved. Thus, a technology called BPMI feedback in which the terminal apparatus notifies the base station apparatus of Best companion PMI (BPMI) representing the linear filter having the best compatibility with the desired linear filter for spatial multiplexing, in addition to the desired linear filter is discussed in NPL 2. This is a method in which the base station apparatus acquires the desired linear filter and the BPMI of which notification is sent from the plurality of terminal apparatuses, and spatially multiplexes the terminal apparatuses of which the desired linear filters thereof are the BPMIs of the opponent terminal apparatuses, and this enables improvement of the opportunity to perform spatial multiplexing of a codebook-based MU-MIMO.
However, the MIMO transmission considered in LTE is 2D-MIMO considering only an antenna gain in the horizontal direction. Meanwhile, for the purpose of improving the opportunity to perform spatial multiplexing in the MU-MIMO described above, 3D-MIMO also considering an antenna gain in the vertical direction is discussed in NPL 3 and the like. If the space resource in the vertical direction is used, more efficient suppression of inter-user interference is possible, such that the improvement of the opportunity to perform spatial multiplexing of MU-MIMO and the improvement of the frequency use efficiency of SU-MIMO can be expected.